The invention relates to a high-pressure discharge lamp comprising a light transmitting discharge vessel sealed in a gas-fight manner and a filling in said discharge vessel comprising a metal compound and a rare gas, wherein said metal compound evaporates during normal operation and decomposes to form condensed metal particles, or droplets, which generate light by incandescent emission.
Such a high-pressure discharge lamp is known from EP-0 420 335 A2.
In the known lamp, the metal compound is selected from among rhenium oxide, and halides and oxyhalides of tungsten, rhenium and tantalum. Since these compounds are generally aggressive to tungsten bodies, said lamp does not comprise electrodes and is excited at a high frequency of between 0.1 MHz and 50 GHz, although such operation requires an expensive control apparatus to provide such high operating frequencies. The lamp has a useful life of several thousands of hours of operation. This is in contrast to a similar lamp having tungsten electrodes, which would have a life of a few hours only. Electrodes would be attacked by the filling and be destroyed, involving the risk of the lamp vessel being destroyed by explosion.
During operation of the lamp the metal compound evaporates and its vapour dissociates in a hotter region of the discharge, where it arrives by convection and/or diffusion. The metal vapour thus formed condenses to form particles or droplets that are heated to incandescence by the discharge. The particles may migrate to regions of lower temperature and be lost by reaction with halogen and/or oxygen to participate in the cyclic process again.
The advantage of this mechanism of light generation is that the melting point of the light emitting metal is no longer the temperature limit of the incandescent body, as it is the case in ordinary electric incandescent lamps. In lamps of the kind mentioned in the opening paragraph, the incandescent bodies are not in the solid state, at a temperature well below their melting point, but in the liquid state, well above their melting point. This is of interest because at the temperatures concerned the amount of light emitted by a black body is proportional to the fifth power of its temperature. Accordingly, the known lamp contains compounds of tungsten, rhenium or tantalum: the metals having the highest melting points. Only osmium melts at a higher temperature than tantalum. Osmium, however, is dangerous, because it is readily oxidised into a highly toxic oxide.
The luminous efficacy of the known lamp, however, varies from poor to moderate, although efficacies have been obtained which are higher than those ever obtained with incandescent lamps. Its colour rendering index generally is rather high, however, not all standard colours make a high contribution to the light generated. As a result, the light generated has a hint of colour, for example, a hint of green.